Indonesia earthquake and soil liquefaction

On 28 September 2018, a 7.5 maginute earthquake struck in the neck of the Minahasa Peninsula, Indonesia. The quake was located 77 km (48 mi) away from the provincial capital Palu and was felt as far away as Samarinda on East Kalimantan and also in Tawau, Malaysia (590 km away, equivalent to the distance from Washington DC to Toronto).

Unlike previous earthquakes/tsunamis events, most of the building collapse were due to liquefaction. Hundreds of buildings collapsed due to liquefaction. In many cases, objects and structures above ground sink into the earth - with some swallowed completely.

What is liquefaction?

Liquefaction occurs when loose sand and silt that is saturated with water behaves like a liquid when shaken by an earthquake. There are 4 condition for the occurrence of liquefaction:

1. Earthquake waves cause water pressures to increase in the sediment and the sand grains to lose contact with each other, leading the sediment to lose strength and behave like a liquid. The soil can loose its ability to support structures, flow down even very gentle slopes, and erupt to the ground surface to form sand boils. Many of these phenomena are accompanied by settlement of the ground surface — usually in uneven patterns that damage buildings, roads and pipelines. Three factors are required for liquefaction to occur:
2. Loose, granular sediment — typically "made" land and beach and stream deposits that are young enough (late Holocene) to be loose.
3. Saturation of the sediment by ground water (water fills the spaces between sand and silt grains). This is an interesting requirement because it defines a seasonality risk of liquefaction. For instance, in much of the San Francisco Bay region the ground water is closest to the surface (saturating the younger sediment) in the Winter/Spring, during and following the "wet season". In 1906, the Bay region was fortunate that the previous wet season had been relatively dry. In 1989, the Loma Prieta earthquake occurred at the end of the dry season in October, when ground water levels are relatively deep beneath the ground surface — still, there was considerable liquefaction-related damage!
4. Strong shaking — all parts of the San Francisco Bay region have the potential to be shaken hard enough for susceptible sediment to liquefy.

When all four conditions are present, the loose material begins to compress under the force of gravity, closing the spaces between the grains. However, the water already occupying the spaces resists the change, and pressure begins to build in the material. Eventually, the pressure rises enough that the grains become buoyant and float in the water. At this point, the soil has completely lost its strength. Soil that was solid begins to act like a liquid.

With lower bearing strength, liquefied soil can no longer support the same amount of weight as it did when it was solid. Objects on the surface, such as buildings, may partially settle or sink into the liquefied soil, causing damage to building foundations and the structure that they support. Soil can remain liquefied for several hours after the earthquake shaking has stopped, although it will gradually solidify and regain bearing strength as the pressure within the material disperses.